1. Field of the Invention
This invention relates to pigmented algae-resistant inorganic granules which match the color of non-algae-resistant inorganic granules to the extent that the difference between the color of the algae-resistant and non-algae-resistant granules cannot be visually detected.
2. Description of Related Art
Roofing granules, both natural and artificially color-coated, find extremely wide use in roofing and siding materials. Important applications are in granular surfaced bituminous roll roofing and asphalt shingles. The granules, which are partially embedded in one surface of asphalt-impregnated and/or asphalt-coated fiber sheet material, form a coating to provide an inherently weather-resistant, fire-resistant, and decorative exterior surface. "Weather-resistant" as used herein includes ultraviolet (UV) radiation-resistant.
Skadulis (U.S. Pat. No. 3,528,842) describes roofing granules which contain an algicide, such as cuprous oxide. As explained by Skadulis, methods have long been known to artificially color roofing granules. This consists primarily of utilizing crushed and screened minerals, in which the granules are coated with a suitable pigment in an inorganic matrix or bond. The bond is usually applied in the form of a soluble silicate solution and is insolubilized either by heat treatment or a combination of heat treatment and chemical action to a substantially water-insoluble state and is strongly adherent to the base granule. In carrying out these methods the pigment is typically uniformly applied to the granular surface with the soluble silicate solution. The soluble silicate binder is converted to an insoluble state by heat treatment and chemical action, as by the addition of clay or an acidic material. This can occur either before, during or after the application of the mixture of the soluble silicate and pigment to the mineral granules, in order to obtain a chemical reaction between the clay or acidic material and the alkaline portion of the silicate.
Skadulis notes that numerous organic chemicals have been used to control or prevent the growth of fungi and algae. Such chemicals cannot, however, be applied to granules before firing because they would be volatilized during firing of the granules and rendered ineffective. Thus, the roofing granule industry has turned to inorganic algicidal compounds such as copper sulfate (CuSO.sub.4) cupric oxide (Cu.sub.2 O), silver iodine (AgI) and lead phosphate Pb.sub.3 (PO.sub.4).sub.2. However, in the case of copper sulfate, the substance is too water soluble and would be leached out of a surface by rainfall and other naturally occurring moisture within a few months. On the other hand, virtually water insoluble compounds such as cupric oxide, silver iodine and lead phosphate proved ineffective due to reasons which were not fully understood but were believed to be insufficient solubility of the materials.
To overcome these problems, Skadulis discovered that substantially water-insoluble copper compounds which had limited solubility in acidic solutions, such as cupric oxide (Cu.sub.2 O) or cupric bromide (CuBr.sub.2) in porous ceramic coatings proved surprisingly effective in maintaining surfaces coated therewith free of algae infestations for long periods of time when incorporated in the coating compositions in very minor amounts. Skadulis notes that in the case of algae-resistant roofing granules, roofs may be made up from shingles coated wholly with such granules or the algae-resistant granules may be mixed or blended with conventional (non-algicidal) granules. However, the color-fastness of algae-resistant and non-algae-resistant granules has been determined to vary, so that after a period of time the end user may still be faced with the prospect of a roof which changes color as it matures, with or without algae growth and its attendant problems. It would be advantageous if algae-resistant granules could be made to have a visual appearance which is indistinguishable from the non-algae-resistant inorganic granules. The present invention is drawn to such algae-resistant inorganic granules, a composite sheet body for roofing and siding which includes such granules, a color-coating composition suitable for preparing algae-resistant insolubilized and unglazed artificially colored coatings, and a method of protecting exterior surfacing from the growth thereon of discoloring microorganisms.
McMahon (U.S. Pat. No. 3,507,676) further describes zinc-compound containing algicidal surfacing granules. The roofing granules contain either zinc metal, zinc oxide (ZnO), or zinc sulfide (ZnS), rather than copper-containing compounds patented by Skadulis. McMahon teaches the use of a pigment grade ZnO in either a first or second coating, with a concentration of about 10 grams ("gms") ZnO per kilogram ("kg") of inorganic roofing granules. McMahon noted that ZnO had been previously used in pigmented ganules as an insolubilization agent or pigment, but in quantities insufficient to be useful for long-term algae-resistance.
Lodge et al. (U.S. Pat. No. 3,255,031) describes the use of a small amount of a borate, such as "borax" and the like, in clay-silicate coating compositions. The addition of about 2 gms borate per kg of inorganic granules apparently allows the temperature of the firing kiln to be reduced below about 450.degree. C., and results in a granule which does not have to be treated with pickling agents to insolubilize the coating. Color benefits such as "lightening" of all pigmented granules and "whitening" of dark-pigmented granules are derived from the use of borate-clay-silicate coatings. Borates are disclosed as being used in a single coating or in the inner or outer coating of a two layer coating system. Skadulis and McMahon also mention that borates may be used in the processes of making their coating formulations.
U.S. Pat. No. 3,888,684 (Little) describes algicidal roofing granules having two fully water-insolubilized silicate-clay coatings each containing algicidal compounds, with the inner insolubilized coating have a greater algicidal content than the outer insolubilized coating. The algicidal compounds employed are zinc algicidals, copper algicidals or mixtures thereof.
As noted by Nelson in U.S. Pat. Nos. 3,894,877 and 3,888,682, the unavailability of a totally satisfactory algicidal roofing granule reflects the continual desire in the roofing industry for a more effective algicidal effect from a roofing granule of ever diminishing incremental cost to achieve such an effect. The use of minimum quantities of metallic algicide to produce a desired level of effectiveness over an extended period of time is, therefore, highly desirable. Nelson also notes that the use of a relatively large amount of metallic algicide frequently requires the incorporation of pigment in the granule coating in amounts in excess of that otherwise required to achieve a desired roofing granule color. Nelson gives an example, that as the amount of cupric oxide employed is increased, the amount of titanium dioxide pigment that must be employed in the granular coating composition to produce a white roofing granule is also generally increased, adding to the overall cost of the algicidal roofing granule. Nelson also notes that a further troublesome aspect of efforts to impart algicidal properties to roofing granules has been the practical necessity for producing such algicidal roofing granules as a separate production operation apart from the production of conventional-non-algicidal, color-coated roofing granules because of the variation in the color coating formulations necessarily required to incorporate the algicidal compound into the color coating. Nelson maintains that the requirements for the production of algicidal roofing granules in accordance with the teachings of Skadulis and McMahon constitute a further detrimental element serving to diminish the prospects for employing algicidal roofing granules despite the genuine need for algae control.
Thus, the Nelson patent was drawn in large part to a process whereby conventional non-algicidal color-coated roofing granules are treated with a mixture of a heavy processing oil and a copper silicate, alone or in combination with other metallic algicides, to provide algicidal colored roofing granules. The algicidal materials are applied, according to the Nelson process, to conventionally color-coated granules by incorporation into the heavy processing oil that is mixed with the color-coated granules, as in conventional post-treatment operations in which lighter processing oils are generally employed, for example, to reduce dust generation when processing the granules. Nelson notes that heavy oils are commonly dark in appearance, and such oils would ordinarily be deemed inappropriate for application to color coded roofing granules as likely tending to mask or adversely effect the desired color of the color coating. However, the use of heavy oils apparently enhanced the adhesion of the algicidal materials to the surface of the granules, minimizing any tendency for loss of algicidal materials from the roofing granules due to wash-off or fall-off of particles as a result of atmospheric weathering or any other cause. Thus, algicidal properties were enhanced, but the problem of matching the color of algicidal and non-algicidal granules remains.
U.S. Pat. Nos. 3,888,176 and 3,888,683 (Horai, Jr. et al.) describe particles similar to those of Nelson which are made by incorporating metallic algicides with the processing oils conventionally employed in the post-treatment of color-coated roofing granules for dust control, i.e., those oils have a viscosity of about 100-500 SUS measured at 100.degree. F.
U.S. Pat. No. 4,092,441 (Myer et al.) discloses a roofing granule treatment process whereby metallic algicides, such as zinc and copper, are sprayed in the form of droplets of molten metal onto the surface of roofing granules or onto the surface of asphalt roofing compositions in which said granules are imbedded. The metal droplets solidify and adhere on the surface of the granules or compositions as relatively fine, randomly distributed, irregular shaped metal globules. During periods of rain or dew upon atmospheric exposure of roofing compositions incorporating therein roofing granules so treated, the oxidized globules become ionized so that metallic algicidal ions are slowly released therefrom and leached over the roofing surface, thus retarding the biological growth of algae and/or fungi over extended periods of time.
Other U.S. Patents of interest include U.S. Pat. Nos. 3,598,627; 3,484,267; 2,111,131; 2,225,867; 3,826,825; 2,981,636; 4,378,408; 4,359,505; 2,070,359; 3,752,696; 3,046,188; 1,720,708; and 3,985,540.
In particular, U.S. Pat. No. 2,111,131 discloses that small but highly effective amounts of zinc oxide may be dissolved in sodium silicate by effecting the dissolution at elevated temperatures and pressures, such as by introducing steam into an autoclave conataining zinc oxide and the silicate. U.S. Pat. No. 2,225,867 discloses that the algicidal effectiveness of copper oxides (especially cuprous oxide) is increased by the presence of Zinc oxide, and that it did not appear to be dependent upon the purity, source, and the like, of the zinc oxide.
Minnesota Mining and Manufacturing Company ("3M") has commercialized since 1990 pigmented roofing granules known under the trade designations "LR-7000" and "LR-7070". These granules have three insolubilized silicate-clay coatings, the first two coatings each having about 35 gm cuprous oxide per kg substrate granules, with the third coating having no cuprous oxide but having pigments to render the desired color. None of the three coatings contains zinc oxide or borate. Assignee's copending application Ser. No. 07/945,128, filed Sep. 15, 1992, describes algicidal and non-algicidal roofing granules similar to the LR-7000 granules, improved by the addition of zinc oxide made by the "French process" in substantially equal proportions to the first and second coatings, and zinc oxide and borate in the third coating. (Zinc oxide made the French process differs from zinc oxide manufacturesd by the "American process". The difference is discussed in the copending application in detail with reference to Kirk-Othmer, Encyclopedia of Chemical Technology, Vol. 24, 3rd Ed. John Wiley & Sons (New York) pp. 854-863. Surprisingly, the granules of the copending application exhibit improved akalinity and adhesion properties over previously known granules employing other grades of zinc oxide, such as pigment grade zinc oxide.
As mentioned above, there still exists, however, even with the improved granules of the above-mentioned copending application, the long standing problems of extending the algae-resistance of roofing granules, color matching algicidal and non-algicidal roofing granules, and reducing the tendency for pigmented algae-resistant granules to color-fade. Therefore, it would be advantageous if pigmented algicidal roofing granules could be developed which meet these long-felt needs.